Quadrature amplitude modulation (QAM) is an intermediate frequency (IF) modulation scheme in which a QAM signal is produced by amplitude modulating two baseband signals, generated independently of each other, with two quadrature carriers, respectively, and adding the resulting signals. The QAM modulation is used to modulate a digital information into a convenient frequency band. This may be to match the spectral band occupied by a signal to the passband of a transmission line, to allow frequency division multiplexing of signals, or to enable signals to be radiated by smaller antennas. QAM has been adopted by the Digital Video Broadcasting (DVB) and Digital Audio Visual Council (DAVIC) and the Data-Over-Cable Service Interface Specification (DOCSIS) standardization bodies for the transmission of digital TV signals over Coaxial, Hybrid Fiber Coaxial (HFC), and Microwave Multi-port Distribution Wireless Systems (MMDS) TV networks.
The QAM modulation scheme exists with a variable number of levels (4, 16, 32, 64, 128, 256, 512, 1024) which provide 2, 4, 5, 6, 7, 8, 9, and 10 Mbit/s/MHz. This offers up to about 42 Mbit/s (QAM-256) over an American 6 MHz CATV channel, and 56 Mbit/s over an 8 MHz European CATV channel. This represents the equivalent of 10 PAL or SECAM TV channels transmitted over the equivalent bandwidth of a single analog TV program, and approximately 2 to 3 High Definition Television (HDTV) programs. Audio and video streams are digitally encoded and mapped into MPEG2 transport stream packets, consisting of 188 bytes.
The bit stream is decomposed into n bits packets. Each packet is mapped into a QAM symbol represented by two components I and Q, (e.g., n=4 bits are mapped into one 16-QAM symbol, n=8 bits are mapped into one 256-QAM symbol). The I and Q components are filtered and modulated using a sine and a cosine wave (carrier) leading to a unique Radio Frequency (RF) spectrum. The I and Q components are usually represented as a constellation which represents the possible discrete values taken over in-phase and quadrature coordinates. The transmitted signal s(t) is given by: EQU s(t)=I cos(2.pi.f.sub.0 t)-Q sin(2.pi.f.sub.0 t),
where f.sub.0 is the center frequency of the RF signal. I and Q components are usually filtered waveforms using raised cosine filtering at the transmitter and the receiver. Thus, the resulting RF spectrum is centered around f.sub.0 and has a bandwidth of R(1+.alpha.), where R is the symbol transmission rate and .alpha. is the roll-off factor of the raised cosine filter. The symbol transmission rate is 1/n.sup.th of the transmission bit rate, since n bits are mapped to one QAM symbol per time unit 1/R.
In order to recover the baseband signals from the modulated carrier, a demodulator is used at the receiving end of the transmission line. The receiver must control the gain of the input amplifier that receives the signal, recover the symbol frequency of the signal, and recover the carrier frequency of the RF signal. After these main functions, a point is received in the I/Q constellation which is the sum of the transmitted QAM symbol and noise that was added over the transmission. The receiver then carries out a threshold decision based on lines situated at half the distance between QAM symbols in order to decide on the most probable sent QAM symbol. From this symbol, the bits are unmapped using the same mapping as in the modulator. Usually, the bits then go through a forward error decoder which corrects possible erroneous decisions on the actual transmitted QAM symbol. The forward error decoder usually contains a de-interleaver whose role is to spread out errors that could have happened in bursts and would have otherwise have been more difficult to correct.
A common measure used to describe the occurrence of errors in a digital data transmission system is the Bit Error Rate (BER). Generally, the BER is estimated by counting the number of bits received in error during a specified interval and taking the ratio of the number of the bits received in error to the total number of bits received. Many QAM systems employ this type of bit error rate measurement to determine and/or correct errors in the transmission system. For example, U.S. Pat. No. 5,163,051 to Biessman et al. discloses a BER test arrangement, composed of two autonomous BER test systems, which effects the full duplex testing of a pair of co-located modems terminating a simulated transmission link by utilizing a single processor to control each independent BER test system and a buffer storage device to post information communicated between the controller processor and each of the test systems. Also, U.S. Pat. No. 5,168,509 to Nakamura et al. discloses a multi-level QAM communication system that uses Reed-Solomon encoders and Reed-Solomon decoders for error correction purposes.
In general, the bit error rate is usually estimated at the receiver by looking at the number of corrected bytes from the forward error correction decoder. Using only this information presents a problem that occurs when a high or bursty bit error rate is present. If the number of errors is greater than the correction capacity of the error correcting code, then the information is invalid and cannot be taken into account by the estimator. In general, however, additional information is present in the transmitted frame that is not encoded by the forward error correction (FEC) encoder, such as synchronization patterns. It would be useful to be able to use this information in order to obtain an accurate BER result in the case of a high or bursty bit error rate.
It is the object of the present invention to provide a dual BER estimator in a QAM demodulator that is based on Reed-Solomon correction and sequence pattern recognition for high bit error rate measurements.
It is a further object of the present invention to provide a dual BER estimator in a QAM demodulator that makes it possible to evaluate the quality of a transmission link even in the case when a high error rate is present.